A Study on the Application of Non-azeotropic Refrigerant Mixtures in High Temperature Heat Pumps

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In comparison with conventional single-component refrigerants, the application of refrigerant mixtures in high temperature heat pumps may result in a higher coefficient of performance, lower condensing pressure, and higher reliability. But it is well known that the boiling and condensation heat transfer coefficients of binary mixtures are reduced due to the mass transfer effects compared with single-component refrigerants. Main purpose of this study is to evaluate the effect of the reduction in heat transfer coefficients on the heat pump performance.Simple models for evaluating the mass transfer effects on boiling heat transfer and condensation heat transfer were developed. In both boiling and condensation heat transfer, mass diffusion layer near the gas-liquid interface causes the increase in heat transfer resistance.In order to confirm the theories mentioned above and to investigate the feasibility of refrigerant mixtures, a heat pump test rig was constructed, which consisted of water-cooled or -heated counterflow heat exchangers and an open type compressor. The COP and the heat transfer performance for the pure refrigerants R12 and R22 and the mixtures R22 with R114 in different compositions were measured. Within the experimental conditions, the reduction in the boiling heat transfer coefficients was more serious than the condensation heat transfer coefficients. The comparison between the theoretical and experimental results of heat transfer coefficients was satisfactorily good. When the heat transfer area of heat exchangers is large enough and the temperature increase of the heat sink fluid at the condenser is large, the COP can increase.